1. Technical Field of the Invention
The invention relates to a sliding element, in particular a piston ring.
2. Related Art
Sliding elements, such as, for example, pistons rings, have running surfaces on which they are in sliding contact with a friction partner. The tribological system is complex and is significantly determined, for example, by the material pairing of the friction partner and the environmental conditions such as, for example, pressure, temperature and surrounding media. Precisely with modern engines, particularly high loads occur, for example, on the piston rings. To ensure and prolong the functionality and lifetime of the components, the running surface properties of the sliding elements can be specifically optimized.
Such optimizations often comprise applying more or less complex layer systems by means of, for example, thermal spraying processes, galvanic processes or thin-layer-technology processes. The primary objective of such layers is to protect against wear and focus is first usually on a high degree of hardness of the layers. Diamond-like carbon layers (diamond-like carbon, DLC) have proven to be particularly hard and tenable layers. These may be varied in many ways, for example by changing their C—C bonding character, the different bonding fractions of the carbon and by the presence or absence of hydrogen and metals in their properties. Often adhesive layers are applied under these wear protection layers, which are supposed to ensure a particularly tenable and strong bonding of the highly-loaded wear protection layer with the base material of the sliding element.
However, the exclusive use of very hard DLC layers leads to some technological problems. On the one hand, the surfaces of these layers must be very smooth so that at a higher surface pressure disruptions do not occur on the surface and so that the layer system does not fail. It can furthermore be shown that, for example, the ring wear and liner wear increase significantly with the roughness of the wear protection layers. Thus, it is necessary to smooth the surfaces of the wear protection layers as much as possible prior to use, however this is associated with extensive technical effort and is very cost-intensive. For example, EP 1 829 986 B1 describes a method how such hard carbon-based layers can be processed by means of bristle-shaped or plate-shaped elements. On the other hand, very hard wear protection layers have unfavorable run-in behavior. Owing to its high degree of hardness, the run-in occurs at the expense of the friction partner which in the run-in phase undergoes increased wear, additionally the formation of scoring and/or burn marks can occur.
In view of the run-in behavior, it can be expedient to provide run-in layers which are softer than the wear protection layers on these. The objective of these layers is to generate a type of tribological “balance” in that the run-in layer is stripped at the first contact with the friction partner and in doing so it performs a reciprocal adjustment of the friction partner. Following the run-in phase, the frictional wear slows down and stabilizes and the hard wear protection layer then ensures long-term favorable friction properties and the lifetime durability.
DE 10 2005 063 123 B3 describes a layer system on a sliding element from the inside to the outside consisting of a wear protection layer, an adhesive layer and a run-in layer of the type Me—C:H, in which hard material particles are also contained, for example WC.
A sliding element is evident from DE 10 2008 042 747 A1 having a coating which from the inside to the outside has an adhesive layer, a PVD layer, optionally a carbon-based layer of the type a-C:H:W, a carbon-based layer of the type a-C:H and a further carbon-based layer of the type a-C:H. It is provided that the outer carbon layer is softer than the underlying carbon layer.